The present invention relates to a process for producing Ni-steels with high toughness having high crack-arresting capability and tensile strength values on the order of 50-100 kgf/mm.sup.2 at low temperature.
In order to cope with the increasing consumption of energy, a great number of tanks are being built for storage of LPG and LNG, and this has led to an increasing demand for steel plates as structural components of cryogenic vessels. Steel plates containing 4.0-10% Ni are used to build cryogenic tanks instead of the conventional austenitic stainless steels. Two of the methods for producing such Ni-containing steels are described in Japanese Patent Publication No. 15215/1971 and Unexamined Published Japanese Patent Application No. 104427/1980. The first reference discloses "a three-stage process of heat treatment consisting of normalizing a low-carbon Ni-steel at a temperature not lower than the Ac.sub.3 transformation point, heating and quenching the steel at temperatures between the Ac.sub.1 and Ac.sub.3 transformation points, and tempering the hardened steel at a temperature not higher than the Ac.sub.1 transformation point". The second reference shows "a process comprising rolling a steel to provide a reduction of 60% or more in the temperature range of 1,100.degree. C. to the Ar.sub.3 transformation point, subsequently holding the rolled steel at a temperature between the Ar.sub.3 and Ar.sub.1 transformation points for a period of 30-60 minutes followed by quenching, and thereafter tempering the hardened steel at a temperature not higher than the Ac.sub.1 transformation point". The Ni-containing steel plates produced by these methods exhibit high strength and superior toughness at cryogenic temperature.
However, with a view to preventing failure of LNG and LPG tanks, industry-wide efforts are being made to ensure even greater safety in cryogenic tanks by employing steel plates of high cryogenic toughness that have high strength, high crack-arresting capability and minimum variations in performance.
The term "crack-arresting capability" means the ability of a steel to stop the progress of brittle cracking occurring in the steel. While many processes are known to be capable of providing an improved crack-arresting capability, two are described here. Unexamined Published Japanese Patent Application No. 100624/1983 discloses "a method comprising rough hot-rolling a Ni-containing steel wherein Nb is combined with selective additions of B, Ti, Cu or Cr, then finish-rolling the steel at a temperature for the dual-phase region, followed by quenching and tempering".
This method depends on hot rolling at a temperature in the dual-phase region for attaining an improved crack-arresting capability. Another prior art method for producing a steel having an improved crack-arresting capability is described in Unexamined Published Japanese Patent Application No. 217629/1983. This method is characterized by controlling the cumulative reduction for rolling in a lower-temperature region, and comprises "heating a Ni-steel slab containing Cr and/or Mo to 1,150.degree. C., then hot-rolling the slab at a temperature of 850.degree. C. or below to impart a cumulative reduction of 60% or more, immediately thereafter water-cooling the rolled slab, following by tempering at a temperature not higher than the Ac.sub.1 transformation point".
These methods are essentially the same as the methods described in Japanese Patent Publication No. 15215/1971 and Unexamined Published Japanese Patent Application No. 104427/1980 that are intended for producing steel plates having improved strength and low-temperature toughness. Each of these methods depends on producing a steel structure with finer grains for taking full advantage of the great ability of the Ni component to stop brittle cracking. The degree of improvement in crack-arresting capability acheived by these methods is not sufficient to be considered satisfactory and only inconsistent results are obtained.